Influenza is an infectious disease caused by RNA viruses of the family Orthomyxoviridae (the influenza viruses). Influenza viruses are classified based on core protein into three genera A, B and C that are further divided into subtypes determined by the viral envelope glycoproteins hemagglutinin (HA) and neuraminidase (NA). Influenza A viruses infect a range of mammalian and avian species, whereas type B and C infections are largely restricted to humans. Only types A and B cause human disease of any concern.
High mutation rates and frequent genetic reassortment of the influenza viruses contribute to great variability of the HA and NA antigens. Minor point mutations causing small changes (“antigenic drift”) occur relatively often. Antigenic drift enables the virus to evade immune recognition, resulting in repeated influenza outbreaks during interpandemic years. Major changes in the HA antigen (“antigenic shift”) are caused by reassortment of genetic material from different influenza A subtypes. Antigenic shifts resulting in new pandemic strains are rare events, occurring through reassortment between animal and human subtypes, for example in co-infected pigs.
Influenza A spreads around the world in seasonal epidemics, resulting in the deaths of between 250,000 and 500,000 people every year, and up to millions in some pandemic years. On average 41,400 people died each year in the United States between 1979 and 2001 from influenza.